1. Field of the Invention
The present invention relates to short circuit protection circuitry for power supplies or power units of telecommunications or data handling apparatus. The invention is particularly applicable to ringing generators, multifrequency ringing generators for telecommunications applications and to power supplies.
2. Description of the Prior Art
Power supplies, telephone ringing generators or any other electrical apparatus which utilizes high voltage and current must be protected against a short circuit placed across its output terminals. One such apparatus requiring such short circuit protection is a telephone ringing generator which typically utilizes -48Vdc and includes means for converting this voltage to a low frequency sinewave of 20 HZ at 100 Vrms. If such apparatus did not have such short circuit protection, it would try to draw an infinite amount of current under this stressful load. In accordance with the known prior art, a self-protection technique is initiated after the output current exceeds a certain threshold.
Presently, there are two well known and widely used techniques for protecting a ringing generator against a short circuit. The first such short circuit protection technique is shown in FIG. 1 and is referred to as Current Limiting. In accordance with this self-protection method, the voltage decreases in order to maintain a constant output current as the impedance of the load approaches zero. During this period of protection, the output of the protected apparatus is no longer in an acceptable region of usage and tremendous energy is expended. The apparatus will return to normal operation only if the impedance of the load returns to a value which is above a fixed threshold.
The second well known prior art short circuit protection technique is shown in FIG. 2, whereby the amount of energy expended under current limiting is reduced. This self-protection scheme is known as current foldback, and it reduces the short circuit current to a fraction of the full load output current. As in the case of current limiting, during the protection period the output of the protected apparatus is no longer in an acceptable region of usage, but considerably less energy is expended. The major problem with this known technique is that for the apparatus to return to normal operation the load must be removed from the output.
An example of a well known telecommunications ring signal generator of the multifrequency type is illustrated by U.S. Pat. No. 4,656,659. An example of a well known current limited subscriber line feed circuit is illustrated by U.S. Pat. No. 4,254,305.